Process for reliquefying a methane-rich fraction

ABSTRACT

A process for reliquefying a methane-rich fraction, in particular boil-off gas, is described. In this process,
         a) a methane-rich fraction is compressed to a pressure at least 20% above the critical pressure of the fraction to be compressed,   b) liquefied and supercooled,   c) depressurized to a pressure in the range from 5 to 20 bar,   d) separated into a gaseous nitrogen-rich fraction and a liquid nitrogen-depleted fraction,   e) the nitrogen-depleted fraction is depressurized to a pressure in the range from 1.1 to 2.0 bar,   f) the gaseous fraction obtained, without being warmed and compressed, is mixed into the methane-rich fraction, and   g) the liquid product fraction obtained in the depressurization of the low-nitrogen fraction has a nitrogen content of ≦1.5 mol %.

The invention relates to a process for reliquefying a methane-richfraction, in particular boil-off gas.

In the following, the term “boil-off gas” refers both to boil-off gasesand also to gas mixtures which have a similar composition; displacementgases which arise, for example, in the loading of LNG into transporttanks on ships or goods vehicles may be mentioned merely by way ofexample.

In the liquefaction of methane-rich gases or boil-off gases, appropriatemeasures for discharging a nitrogen-rich fraction are required above acertain nitrogen content in order to limit the nitrogen content of theliquefied natural gas (LNG) to usually 1 mol %.

U.S. Pat. No. 5,036,671 discloses a method of discharging anitrogen-rich fraction, in which gas streams which have a significantlyincreased content of nitrogen compared to the crude gas are taken off atthe cold end of the liquefaction process via one or more separators.These gas streams are generally compressed, optionally partlyrecirculated to the crude gas and usually used as fuel gas. In theliquefaction process described in U.S. Pat. No. 5,036,671, the boil-offgas flowing from the LNG tank located downstream of the liquefactionprocess is warmed and compressed at approximately ambient temperature.

Since the operating pressure in such LNG tanks is normally onlyslightly, typically 50 mbar, above ambient pressure, there is a highprobability of generating subatmospheric pressure in the compressor inthe warm-intake compression of the boil-off gas. This can lead to entryof air and thus oxygen and thus represent a safety risk.

It is an object of the present invention to propose a process of thetype in question for reliquefying a methane-rich fraction, which avoidsthe abovementioned disadvantages.

To achieve this object, a process of the type in question forreliquefying a methane-rich fraction, in which

-   -   a) the methane-rich fraction is compressed to a pressure which        is at least 20% above the critical pressure of the fraction to        be compressed,    -   b) liquefied and supercooled,    -   c) depressurized to a pressure in the range from 5 to 20 bar and    -   d) separated into a gaseous nitrogen-rich fraction and a liquid        nitrogen-depleted fraction and    -   e) the nitrogen-depleted fraction is depressurized to a pressure        in the range from 1.1 to 2.0 bar,    -   f) where the gaseous fraction obtained is, without being warmed        and compressed, mixed into the methane-rich fraction and    -   g) the liquid product fraction obtained in the depressurization        of the low-nitrogen fraction has a nitrogen content of ≦1.5 mol        %, is proposed.

If the liquefaction and supercooling of the methane-rich fraction arecarried out against at least one refrigerant circuit and/or at least onerefrigerant mixture circuit and this/these has/have at least one circuitcompressor the pressure to which the methane-rich fraction iscompressed, the pressure to which the liquefied and supercooledmethane-rich fraction is depressurized and the temperature to which themethane-rich fraction is cooled are selected or varied according to theinvention in such a way that

-   -   the drive power of the compressor used for compressing the        methane-rich fraction and the drive power of the circuit        compressor(s) are shifted relative to one another without the        total power changing by more than ±5% or    -   the drive power of the compressor used for compressing the        methane-rich fraction and the drive power of the circuit        compressor(s) are shifted relative to one another in such a way        that a division of the total power in the range from 30/70 to        70/30 is achieved.

Further advantageous embodiments of the process of the invention forreliquefying a methane-rich fraction, which are subject matter of thedependent claims, are characterized in that

-   -   the methane-rich fraction is compressed to a pressure which is        at least 30% above the critical pressure of the fraction to be        compressed,    -   the liquefied and supercooled methane-rich fraction is        depressurized to a pressure in the range from 7 to 15 bar and/or    -   the nitrogen-depleted fraction is depressurized to a pressure in        the range from 1.2 to 1.8 bar.

The process of the invention for reliquefying a methane-rich faction andalso further advantageous embodiments of this process will beillustrated below with the aid of the example shown in FIG. 1.

The methane-rich fraction 1 to be reliquefied is compressed in thesingle-stage or multistage compressor unit C1 to a pressure which is atleast 20%, preferably at least 30%, above the critical pressure of themethane-rich fraction 1 to be reliquefied. In this way, two-phasestreams of the methane-rich fraction 1 to be reliquefied are avoided inthe heat exchanger(s) of the subsequent liquefaction stage.

According to the invention, the methane-rich fraction 1 to bereliquefied is not warmed before being compressed in C1. Owing to thecompression in C1, the methane-rich fraction to be reliquefied is heatedto a temperature above that of the surroundings, and it is thereforecooled to approximately ambient temperature against cooling water or airin the heat exchanger E1.

In the heat exchanger E2, the compressed methane-rich fraction 2 iscooled to a temperature in the range from −100 to −140° C., preferablyfrom −110 to −130° C., and thereby liquefied and supercooled.

The cooling of the compressed methane-rich fraction can in principle becarried out against any refrigerant circuit or refrigerant mixturecircuit or combinations of these. The refrigerant mixture circuit shownin FIG. 1 is merely one of the many possible variants. The heatexchanger E2 shown in FIG. 1 can in reality be formed by a plurality ofseparate heat exchangers and/or heat exchanger sections. It ispreferably configured as helically coiled heat exchanger having twobundles or as soldered plate exchanger.

After liquefaction and supercooling, the methane-rich fraction 3 takenoff from the heat exchanger E2 is depressurized in the valve V1 to apressure in the range from 5 to 20 bar, preferably from 7 to 15 bar. Thegaseous, nitrogen-rich fraction 4 obtained here is taken off at the topof the separator D1 located downstream of the valve V1, warmed in theheat exchanger E2 against the methane-rich fraction 2 to be cooled, withthis warming being optional. The warmed nitrogen-rich fraction 5 is, ifdesired, subsequently compressed in one or more stages C2 and passed vialine 6 to further use, for example as fuel. This nitrogen-rich gas 5preferably has a pressure in the range from 5 to 20 bar, in particularfrom 7 to 15 bar. It is thus, for example, directly suitable for firingsteam-generating boilers. When used as fuel gas in gas turbines, theoutlay for compression is significantly reduced compared to the priorart in which the initial pressure is a lower tank pressure.

The liquid nitrogen-depleted fraction 7 obtained in the separator D1after depressurization is depressurized in the valve V2 to a pressure inthe range from 1.1 to 2.0 bar, preferably from 1.2 to 1.8 bar. Thegaseous fraction obtained in this depressurization is taken off via line8 from the top of the separator D2 and, without warming, mixed into themethane-rich fraction 1 to be compressed. The liquid fraction obtainedat the bottom of the separator D2 represents the liquefied natural gasproduct (LNG); this has a nitrogen content of ≦1.5 mol %.

Owing to the cold intake of the fractions or gas mixtures 1 and 8 to becompressed in the compressor stage C1, the safety risk mentioned at theoutset which exists in the case of warm-intake compression of boil-offgases can be effectively prevented. Undesirable and dangerous entry ofair and thus oxygen into the compressor C1 is thus ruled out.

Owing to the recirculation of the gaseous fraction 8 obtained after thesecond decompression V2 to the methane-rich fraction 1 to be compressed,the amount of LNG product can be increased, which is advantageous interms of costs, and the total energy consumption can be reduced.

A process alternative which is not shown in FIG. 1 is to replace theseparator D1 by a stripper. In this, the methane-rich fraction 3 whichhas been depressurized in the valve V1 is stripped of nitrogen frombelow by a substream of the methane-rich fraction 2 to be cooled oversuitable internals such as packing and/or trays. As the requiredstripping gas, a substream of the methane-rich fraction 2 to be cooledis drawn in either between the heat exchangers E1 and E2 or, in anembodiment as helically coiled heat exchanger having two bundles,between the bundles.

As mentioned above, cooling and liquefaction of the methane-richfraction 2 is effected in the heat exchanger E2 against a refrigerantmixture circuit shown merely by way of example. The refrigerant mixtureis, after warming and vaporization in the heat exchanger E2 against themethane-rich fraction 2 to be cooled, conveyed via line 10 into aseparator D3 located upstream of a two-stage compressor unit C3. This isin the interests of the safety of the compressor unit C3, since liquidparticles entrained in the refrigerant mixture precipitate therein.

The refrigerant mixture to be compressed is conveyed from the top of theseparator D3 via line 11 to the compressor unit C3 and compressed in thefirst stage thereof to an intermediate pressure. After cooling in theintermediate cooler E3, the refrigerant mixture which has beencompressed to the intermediate pressure is fed via line 12 to a secondseparator D4. The relatively low-boiling refrigerant mixture fractiontaken off from the top of the latter is fed via line 13 to the secondcompressor stage of the compressor unit C3 and compressed in this to thedesired final pressure. This refrigerant mixture fraction issubsequently cooled in the after-cooler E4 and fed via line 15 to athird separator D5.

The liquid fraction obtained in this separator D5 is recirculated vialine 16 and valve V3 to a point upstream of the second separator D4. Therelatively low-boiling refrigerant mixture fraction taken off from thetop of the third separator D5 via line 17 is, after mixing with theliquid relatively high-boiling refrigerant mixture fraction 14 taken offfrom the bottom of the second separator D4, conveyed via line 18 throughthe heat exchanger E2. In order to be able to “bridge” the pressuredifferences in the lines 14 and 17, a pump P is provided in the line 14.

The refrigerant mixture 18 which has been cooled, liquefied andsupercooled against itself in the heat exchanger E2 is, after havingbeen taken off from the heat exchanger E2, depressurized in the valve V4so as to generate cold and subsequently conveyed via line 19 through theheat exchanger E2 again in countercurrent to the methane-rich fraction 2to be liquefied.

In the process of the invention for reliquefying a methane-richfraction, the powers of the feed gas compressor C1 and of therefrigeration circuit compressor C3 can be shifted relative to oneanother by suitable choice of the pressures downstream of the compressorunit C1 and the valve V1 and of the temperature of the cooledmethane-rich fraction 3 before depressurization in the valve V1, withoutthe total power being appreciably, meaning an increase or decrease of±5%, changed.

It is possible, advantageously, to adapt the required powers of thedrives A and B of the compressors/compressor units C1 and C3 to such anextent that drives (gas turbines, steam turbines and/or electric motors)having the same power can be used. This simplification is of greateconomic advantage. Such a redistribution of the drive powers of thefeed gas compressor C1 and the refrigeration circuit compressor C3 isneither known from the prior art nor rendered obvious thereby.

The amount of gas taken off at the top of the separator D1 can be keptconstant by varying the pressure in the separator D1. This results in avariable amount of gaseous fraction 8 which is recirculated from theseparator D2 to the suction side of the feed gas compressor C1.

As mentioned, a preferred redistribution between thecompressor/compressor units C1 and C3 leads to equal drive powers.Instead of this 50/50 solution, any other distribution in the range from30/70 to 70/30 can be achieved. The solution preferred in each casedepends, for example, on the power steps of customary drives (gasturbines).

1. Process for reliquefying a methane-rich fraction, in particularboil-off gas, wherein a) the methane-rich fraction (1) is compressed(C1) to a pressure which is at least 20% above the critical pressure ofthe fraction to be compressed, b) liquefied and supercooled (E2), c)depressurized (V1) to a pressure in the range from 5 to 20 bar and d)separated into a gaseous nitrogen-rich fraction (4) and a liquidnitrogen-depleted fraction (7) and e) the nitrogen-depleted fraction (7)is depressurized (V2) to a pressure in the range from 1.1 to 2.0 bar, f)where the gaseous fraction (8) obtained is, without being warmed andcompressed, mixed into the methane-rich fraction (1) and g) the liquidproduct fraction (9) obtained in the depressurization of thelow-nitrogen fraction has a nitrogen content of ≦1.5 mol %.
 2. Processaccording to claim 1, where the liquefaction and supercooling (E2) ofthe methane-rich fraction (1) are carried out against at least onerefrigerant circuit and/or at least one refrigerant mixture circuit andthis/these has/have at least one circuit compressor (C3), characterizedin that the pressure to which the methane-rich fraction (1) iscompressed (C1), the pressure to which the liquefied and supercooledmethane-rich fraction (3) is depressurized (V1) and the temperature towhich the methane-rich fraction (2) is cooled are selected or varied insuch a way that the drive power of the compressor (C1) used forcompressing the methane-rich fraction (1) and the drive power of thecircuit compressor(s) (C3) are shifted relative to one another withoutthe total power changing by more than ±5% or the drive power of thecompressor (C1) used for compressing the methane-rich fraction (1) andthe drive power of the circuit compressor(s) (C3) are shifted relativeto one another in such a way that a division of the total power in therange from 30/70 to 70/30 is achieved.
 3. Process according to claim 1,characterized in that the methane-rich fraction (1) is compressed (C1)to a pressure which is at least 30% above the critical pressure of thefraction to be compressed.
 4. Process according to claim 1,characterized in that the liquefied and supercooled methane-richfraction (3) is depressurized (V1) to a pressure in the range from 7 to15 bar.
 5. Process according to claim 1, characterized in that thenitrogen-depleted fraction (6) is depressurized (V2) to a pressure inthe range from 1.2 to 1.8 bar.